Well it's all according to how you look at it...Redemption was different enough from Revolver that they didn't call it Red Dead Revolver 2, so if people want a sequel to the open world version of the Red Dead series, they'd be right in calling it Red Dead Redemption 2. I honestly think that it only goes one of two ways, either they keep the Red Dead & add a different name like Revenge or they simply call it Red Dead Redemption 2. There's absolutely no chance of Rockstar naming it as a 3 as it was basically a wholesale reboot from the first to second. RDR 2 is the more sensible name IMO.

Was Elder Scrolls Online called Elder Scrolls 6? Red Dead Revolver is as different from Redemption as Skyrim is from ESO, they may share a universe but they are different enough to warrant different numbering conventions.